It's probably because it's interesting for me to do so, to see others' views. When I disagree, I tend to enjoy it if people come back and argue. I don't really do it because I care whether the other person comes to see my point of view - although it is more fun when they do, of course. Maybe some people enjoy yelling things such as, "u r a fag!!!!", although personally I can't say I understand those people...
It's useless to try and convince them otherwise for the most part - I don't think they'd listen to you in the first place.
As for the essay itself, I just wish more people would consciously THINK about how they talk to others. It would make the internet and the world a nicer place. (Oh, and that first part, DH0, seemed to me as if it was addressed directly towards Jeff's of Coding Horror response to PG's last essay. But I can't say I disagree - I was actually rather shocked to see such a, umm, 'vehement', response. Another instance of what people probably wouldn't say in person.)